By Sergey Karutin, GLONASS designer general; Nicolay Testoedov, Director General, SC Information Satellite Systems; and Andrey Tulin, Director General, SC Russian Space Systems
This year has marked the 35th anniversary of the first GLONASS launch. During these years, the world has made great strides through high tech, and now no modern society can progress without satellite-based navigation.
Today’s urban resident can hardly do without a smartphone planning his route through traffic, determining the paid parking site location or getting a reminder of parking session completion once he has left the parking lot.
The search for the nearest pharmacy, gas station, restaurant or any other point of interest is of vital necessity today. The growing dependence of modern society on navigation signals-in-space increases the responsibilities of GNSS providers. At the same time, users long for simplicity in getting quality services. That is why this year the GLONASS team is going to set up its most ambitious program: improving the quality of the GLONASS services at a user level.
The traditional GLONASS conception of signal-in-space accuracy is now being augmented by the user level performance estimation. Due to the fact that the signal propagation environment contributes a lot to the positioning error budget, it is obvious that users need information that would reduce the influence of signal propagation path on the positioning accuracy.
Glonass-M satellites currently form the core of the GLONASS constellation, and with six ground spares now in stock, they will continue to do so for at least the next eight years. Therefore, in 2018 the new edition of L1 and L2 FDMA Interface Control Documents are to be published which will include the ionospheric and tropospheric models recommended in the recently released GLONASS CDMA Signals ICDs.
Glonass-K2 satellite (artist’s rendering).
We plan to use the spare bits within the navigation superframe of FDMA signals to transmit ionospheric parameters described in the General Description of the GLObal NAvigation Satellite System with the Code Division Multiple Access Signals ICD.
Studies being performed demonstrate up to 70 percent reduction in impact of ionospheric refraction when using the adaptive model transmitted by the three parameters: the numerical factor for the peak TEC (Total Electron Content) of F2 ionosphere layer, the solar activity index and the daily geomagnetic activity index. In the new CDMA signal message, these parameters are initially provided.
To enable the unanimity of technologies for reducing the hydrostatic component of the tropospheric delay, which accounts for 80 percent of its value, the both FDMA and CDMA Signals ICDs will include the latitudinal tropospheric model based on the preliminary set tabular values.
The preliminary design review for the technical baseline of the fourth-generation Glonass-K2 satellite has been passed this year. The new cubic arrangement of the platform enables mitigation of unmodeled forces and transition of propellant tank to the satellite’s center of mass.
This provides for the relative position constancy for the satellite’s center of mass and the satellite’s antenna phase center during the satellite’s lifetime. This platform arrangement also accommodates the whole ensemble of navigation signals (both CDMA and FDMA) on the single phased-array antenna system.
Glonass-K2 is equipped with the new atomic frequency standard composed of the legacy quantum frequency standard based on the cesium beam tube and the passive hydrogen maser. The miniature PHM with the relative daily stability of 5×10-15 will be installed onboard the satellite to be launched in 2020.
Introduction of the new satellite will enable a new constellation sustainment strategy — through the both dual launches by Angara-A5 launcher from Vostochny and single launches by Soyuz from Plesetsk — to provide on-demand replenishment of the constellation.
By 2020, when we celebrate the 25th anniversary of GLONASS full operational capability, all the efforts mentioned above will offer new quality of services to GLONASS users prioritized as per their needs.
We just went to press with the December issue of GPS World magazine, its cover story consisting of personal essays from the directing officers and architects of each global satnav system.
You’ll have to wait for your copy to arrive in the mail or the digital edition to land in your inbox to read those articles in full, but I’ll give you a sneak preview here — just enough to springboard my hypothesis that, yes, system operators build great systems, but they are still subject to the whims of their national governments.
And those governments are operating on increasingly divergent agendas that will bring consequences upon the respective GNSS industries.
The plots of the four articles by the effective CEOs of each GNSS are the same: what innovations were accomplished in 2017, and what new features to look for in 2018. But the themes differ. If you reflect at the end of each article, try to read between the lines, divine what message seems most important to the author — then distinctions surface.
In our December pages we hear from:
Col. Steve Whitney
Col. Steven Whitney, Director, Global Positioning Systems Directorate:
“The Air Force is already exploring new, emerging technologies and capabilities to even further advance the robustness of the GPS enterprise, with the vision that GPS will always remain the Gold Standard for the worldwide PNT community.”
Sergey Karutin, GLONASS designer general; Nicolay Testoedov, Director General, SC Information Satellite Systems; and Andrey Tulin, Director General, SC Russian Space Systems:
“The traditional GLONASS conception of signal-in-space accuracy is now being augmented by the user level performance estimation. Due to the fact that the signal propagation environment contributes a lot to the positioning error budget, it is obvious that users need information that would reduce the influence of signal propagation path on the positioning accuracy.”
From left: Sergey Karutin, Nicolay Testoedov and Andrey TulinPaul Verhoef
Paul Verhoef, Director of the Galileo Programme and Navigation-related Activities, European Space Agency:
“The world of mobile-device LBS is going to change in 2018, and it will be due to the availability of Galileo. It will not be the first time the partnership of ESA, the European Commission (EC) and the GSA has made a service available that has changed the nature of the marketplace. “
Changfeng Yang, Chief Architect of BeiDou Navigation Satellite System:
“BDS-related products have gained access to the markets of more than 70 countries and regions, more than 30 of which are along the (land-based) Belt and (maritime) Road (in line with the Belt and Road Initiative). Through joint applications with other compatible navigation satellite systems, BDS provides global users with diversified choices for better application experience.”
Changfeng Yang
I’m not suggesting that the directors of each satnav system are trying to accomplish different things. All share the goal of providing the highest quality product and service. I posit that the hands above these guiding hands, atop the top — that is, the national governments paying for each system and directing the directors — do indeed have different priorities. Accuracy is not all that matters on the international scene, taken as a whole. Sound economies, vigorous markets, vibrant technologies, tradeable-upon intellectual property and highly trained corps of engineers all count for as much.
The respective governments’ priorities, which encompass much more than GNSS but surely have an effect upon it, may produce differing results for industry, markets and users. In that light I would bring to your attention a November 14 op-ed column by Thomas Friedman in the New York Times. I’ll return to this in a moment.
The most marked contrast in the “theme” or GNSS subtext of our December essays appears between the United States on the one hand and Europe and China on the other.
GPS appears focused on maintaining the Gold Standard of signals and on beefing up security, particularly for the military. Innovations such as the L2C signal, drawn up with significant if not predominant civil-sector input, will bring important growth and expansion of capabilities. But aside from that, it appears that industry can be left to itself to take care of innovation once the space signal hits the Earth’s surface.
GLONASS communicates a desire to burnish its reputation for accuracy and relevance. Once pre-eminently second among GNSS, it need to bootstrap itself back into competition to secure its increasingly tenuous toeholds in foreign markets.
Galileo and BeiDou project clear messages of working closely with industry sectors to encourage and intensify use. For the governments of Europe and China, GNSS is an economic tool, not merely a political one.
Now for Friedman’s thoughts on the international scene, quickly excerpted in their most relevant glimpses on the GNSS landscape:
“We’re going through a change in the “climate” of globalization: from an interconnected world to an interdependent one; from a world of walls, where you build your wealth by hoarding resources, to a world of webs, where you thrive by connecting your citizens to the most flows of ideas, trade, innovation and education.”
[….]
“In response to a more interdependent world, China is deepening its trade ties to all the fast-growing Asian markets around it through its “One Belt, One Road” project.”
[….]
“China has embarked on a plan called “Made in China 2025” that’s plowing government funds and research into commercializing 10 strategic industries while creating regulations and swiping intellectual property from abroad to make them all grow faster. These industries include electric vehicles, new materials, artificial intelligence, integrated circuits, biopharmacy, quantum computing, 5G mobile communications, and robotics.”
The European Union is actively and aggressively pursuing many projects and mandates similar to those underway in China.
The U.S., not so much. Not even near. In an environment in which it currently imports engineers to fill yawning vacancies in its high-tech work force, how long before it is reduced to importing technology as well, and at what cost?
Hope to See You There. In early of 2018, I will attend both events below, and look forward to talking with as many readers as possible at each event.
Join a full day of expert presentations and discussions on the opportunities and challenges (technical, commercial, ethical and legal) associated with developing fully autonomous systems that are cognizant and trustworthy for safety-critical applications. Free; sponsored by the Institute of Navigation. Speakers from the National Science Foundation, Department of Transportation, Air Force Research Laboratory, Top Flight Technologies, University of California-Santa Barbara, Santa Clara University, The Ohio State University and more.
This three-day international conference focuses on the latest developments in satellite-based navigation, gathering high-ranking speakers from industry, science and governments for a broad overview and differing perspectives. Topics include status and real-world results of Galileo; modernization of GPS, GLONASS and BeiDou; developments of QZSS and NavIC; the need for GNSS authentication; civil use of Galileo Public Regulated Service; legal aspects of GNSS; and autonomy within a single GNSS — still possible?
A review of the history of the GLONASS program, its current status and an overview of the plans for the immediate future of the satellite constellation, its navigation signals and the ground support network.
English versions of the GLONASS CDMA interface control documents are now available. See Further Reading.
Richard Langley
On Oct. 12, 1982, the Soviet Union launched the first GLONASS satellite. Whether in reaction to the development of GPS or simply to fulfill the requirement for a system with similar capabilities for its armed forces, the Soviet Union began the development of the Global’naya Navigatsionnaya Sputnikovaya Sistema or Global Navigation Satellite System in 1976 just three years after the start of the GPS program. The first test satellite, code-named Kosmos 1413, was accompanied by two dummy or ballast satellites with the same approximate mass since the Soviet Union was already planning to launch three GLONASS satellites at a time with its powerful rockets to save on launch costs.
But because of launch failures and the characteristically brief lives of the satellites, a further 70 satellites were launched before a fully populated constellation of 24 functioning satellites (providing full operational capability or FOC) was achieved in early 1996. Unfortunately, the full constellation was short-lived. Russia’s economic difficulties following the dismantling of the Soviet Union hurt GLONASS. Funds were not available, and by 2002 the constellation had dropped to as few as seven satellites, with only six available during maintenance operations! But Russia’s fortunes turned around, and with support from the Russian hierarchy, GLONASS was reborn. Longer-lived satellites were launched, as many as six per year, and slowly but surely a full constellation of 24 satellites returned. And on Dec. 8, 2011, FOC was again achieved and has been subsequently more or less maintained — the system has even operated sometimes with in-orbit spares.
While GLONASS-only and survey-grade dual-system GPS/GLONASS receivers have been around for more than a decade, manufacturers took notice of GLONASS’s rebirth and began producing chips and receivers with GLONASS capability for the consumer market. In 2011, Garmin released handheld receivers supporting both GPS and GLONASS. In the same year, various cell-phone manufacturers started offering GLONASS capability with their embedded positioning modules. The early GPS/GLONASS receivers paved the way for the multi-GNSS receivers we have today, with their capability to track not just GPS and GLONASS satellites but those of the European Galileo and Chinese BeiDou systems, as well as those of the Japanese Quasi-Zenith Satellite System (not to mention the satellites of the satellite-based augmentation systems).
I documented the development of GLONASS in this column back in July 1997, and a team of authors from the Joint Stock Company Russian Space Systems discussed the plans for modernizing GLONASS in an April 2011 article. An update is overdue. So, in this article, I will briefly review the history of the GLONASS program, discuss its current status, and overview the plans for the immediate future of the satellite constellation, its navigation signals and the ground support network.
EARLY YEARS, PRESENT DAY
During the Cold War, information about GLONASS was scarce. Apart from the general characteristics of the satellite orbits and the frequencies used for transmitting the navigation signals, the Ministry of Defence of the Soviet Union revealed little else. However, sleuthing by Professor Peter Daly and his students at the University of Leeds provided some details about the signals’ structure. With the advent of glasnost and perestroika, and the eventual demise of the Soviet Union, information about GLONASS became more readily available. Eventually, the Russians released the Interface Control Document (ICD). This document, similar in structure to the Navstar GPS Space Segment/Navigation User Interfaces ICD-GPS-200, describes the system, its components, and the structure of the signal and the navigation message intended for civil use. Its latest version was published in 2016, but so far this version is only publicly available in Russian.
Satellites and Signals. Six models of GLONASS satellites (also known as Uragan, Russian for Hurricane) have been launched so far. Russia (actually the former Soviet Union) launched the first 10 satellites, called Block I, between October 1982 and May 1985. It sent up six Block IIa satellites between May 1985 and September 1986 and 12 Block IIb satellites between Apri1 1987 and May 1988, of which six were lost because of launch-vehicle-related failures. The fourth model was the Block IIv (v is the English transliteration of the Russian alphabet’s third letter). By the end of 2005, the Russians had deployed 60 Block IIvs. Each subsequent satellite generation contained equipment enhancements and also achieved longer lifetimes.
A prototype GLONASS-M (for Modernized) satellite was launched on Dec. 1, 2001, along with two Block IIvs with the first two production GLONASS-M satellites included in the triplet launches of Dec. 10, 2003, and Dec. 26, 2004. Two GLONASS-M satellites were included in the triplet launch of Dec. 25, 2005. The new design offered many improvements, including better onboard electronics, a longer lifetime, an L2 civil signal, and an improved navigation message. Like earlier versions, the GLONASS-M spacecraft still used a pressurized, hermetically sealed cylinder for the electronics.
FIGURE 1. Image from Reshetnev Information Satellite Systems, manufacturer of the GLONASS satellites, celebrating the 35th anniversary of the launch of the first GLONASS satellite in 1982 (“35 years of service to the world”).
All GLONASS satellites launched since December 2005 have been GLONASS-M satellites with the exception of two GLONASS-K1 (sometimes referred to as just GLONASS-K) satellites, launched on Feb. 26, 2011, and Nov. 30, 2014. GLONASS-K1 satellites are markedly different from their predecessors. They are lighter, use an unpressurized housing (similar to that of GPS satellites), have improved clock stability and a longer, 10-year design life. They also include, for the first time, code-division-multiple-access (CDMA) signals on a third frequency accompanying the legacy frequency-division-multiple-access signals (I’ll discuss these shortly). All of the GLONASS satellites have been manufactured by the Joint Stock Company Reshetnev Information Satellite Systems, located in Zheleznogorsk near Krasnoyarsk in central Siberia, and named after Mikhail Fyodorovich Reshetnev, the founding general director and chief designer. The Reshetnev company was formerly known as the Scientific Production Association of Applied Mechanics (Nauchno Proizvodstvennoe Ob”edinenie Prikladnoi Mekaniki or NPO PM). The Roscosmos State Corporation for Space Activities (formerly the Federal Space Agency), commonly known as Roscosmos, is the governmental body responsible for GLONASS.
FIGURE 1 includes artist’s images of the initial GLONASS, GLONASS-M and GLONASS-K1 satellites.
GLONASS satellite orbits are arrayed in three planes, separated from one another in right ascension of ascending node by 120 degrees, with eight satellites in each plane. The satellites within a plane are equally spaced, separated in argument of latitude by 45 degrees. Satellites in adjoining planes are shifted in argument of latitude by 15 degrees. The satellites are placed into nominally circular orbits with a target inclination of 64.8 degrees and semimajor axis of approximately 25,510 kilometers, giving them an orbital period of about 675.8 minutes. These satellites have ground tracks that repeat every 17 orbits or eight sidereal days. The GLONASS orbit planes are numbered 1–3 and contain orbital slots 1–8, 9–16 and 17–24, respectively.
FIGURE 2 shows the status of the constellation on Oct. 17, 2017. The orbital slot number (also called almanac slot) and frequency channel (discussed below) are given in parentheses. The recently launched GLONASS 752 was set healthy on Oct. 16, 2017, resulting in a fully operational 24-satellite constellation. All of the satellites are standard GLONASS-M satellites except for GLONASS 755, which includes a transmitter for the new third frequency, and GLONASS 701K and 702K. These last two are GLONASS-K1 satellites, with 702K operational while 701K is undergoing flight tests. The “K” isn’t part of the official GLONASS number but has been added to avoid ambiguity. A GLONASS-M satellite launched on Dec. 10, 2003, was also called GLONASS 701. Similarly, the International GNSS Service (IGS) refers to GLONASS 701K and 702K as 801 and 802, respectively. IGS also designates GLONASS 751 as GLONASS 851 to prevent confusion with Kosmos 2080, a GLONASS-IIv satellite launched on May 19, 1990, and also called GLONASS 751. And it designates GLONASS 753 as GLONASS 853 to prevent confusion with Kosmos 2140, a GLONASS-IIv satellite launched on April 14, 1991, and also called GLONASS 751.
FIGURE 2. Status of GLONASS constellation on Oct. 17, 2017. A green square identifies the location of a healthy satellite and orange, a test satellite. Orbital slot numbers and frequency channels are given in parentheses.
The satellites have been traditionally launched three at a time by Proton boosters from the Baikonur Cosmodrome near Leninsk in Kazakhstan. However, starting with the launch of the first GLONASS-K1 satellite, several GLONASS satellites have been launched singly on Soyuz rockets from the Plesetsk Cosmodrome north of Moscow.
Unlike GPS and the other GNSSs, GLONASS uses FDMA rather than CDMA for its legacy signals. Originally, the system transmitted the signals within two bands: Ll, 1602.0–1615.5 MHz, and L2, 1246.0–1256.5 MHz, at frequencies spaced by 0.5625 MHz at L1 and by 0.4375 MHz at L2:
L1k = 1602. + 0.5625k (MHz)
L2k = 1246. + 0.4375k (MHz)
This arrangement provided 25 channels, so that each satellite in the full 24-satellite constellation could be assigned a unique frequency (with the remaining channel reserved for testing). Some of the GLONASS transmissions initially caused interference to radio astronomers, who study very weak natural radio emissions in the vicinity of the GLONASS frequencies. Radio astronomers use the frequency bands of 1610.6–1613.8 and 1660–1670 MHz to observe the spectral emissions from hydroxyl radical clouds in interstellar space, and the International Telecommunication Union (ITU) has afforded them primary user status for this spectrum space. Also, ITU has allocated the 1610–1626.5-MHz band to operators of low-Earth-orbiting mobile communications satellites. As a result, the GLONASS authorities decided to reduce the number of frequencies used by the satellites and shift the bands to slightly lower frequencies.
The system now uses only 14 primary frequency channels with k values ranging from –7 to +6, including two channels for testing purposes (currently –5 and –6). (The +7 channel has also been used in the past for testing purposes.) How can 24 satellites get by with only 14 channels? The solution is for antipodal satellites — satellites in the same orbit plane separated by 180 degrees in argument of latitude — to share the same channel. This approach is quite feasible because a user at any location on Earth will never simultaneously receive the signals from such a pair of satellites. The move to the new frequency assignments started in September 1993.
Like the legacy GPS signals, the GLONASS signals include two pseudorandom noise (PRN) ranging codes: ST (for Standartnaya Tochnost or Standard Precision) and VT (for Visokaya Tochnost or High Precision) similar to the GPS C/A- and P-codes, respectively (but at half the chipping rates), modulated onto the L1 and L2 carriers.
As with GPS, GLONASS transmits the high-precision code on both L1 and L2. But, unlike the GPS satellites, the GLONASS standard-precision code has also been transmitted on the L2 frequencies beginning with the GLONASS-M satellites. (A separate civil code, L2C, has been added to the GPS L2 signal transmitted by Block IIR-M and subsequent satellites.) The GLONASS ST code is 511 chips long with a rate of 511 kilochips per second, giving a repetition interval of 1 millisecond. The VT-code is 33,554,432 chips long with a rate of 5.11 megachips per second. The code sequence is truncated to give a repetition interval of 1 second. Unlike GPS satellites, all GLONASS satellites transmit the same codes. They derive signal timing and frequencies from one of the onboard atomic frequency standards (AFSs) operating at 5 MHz. The various GLONASS satellite series since Block II through to the GLONASS-M series have three cesium AFSs on each satellite. The transmitted signals are right-hand circularly polarized, like GPS signals, and have comparable signal strengths.
Navigation Message. Like GPS and the other GNSSs, the GLONASS signals also contain navigation messages providing satellite orbit, clock and other information. Separate 50-bits-per-second navigation messages are modulo-2 added to the ST- and VT-codes. The ST-code message includes satellite clock epoch and rate offsets from GLONASS System Time; the satellite ephemeris given in terms of the satellite position, velocity and acceleration vectors at a reference epoch; and additional information such as synchronization bits, data age, satellite health, offset of GLONASS System Time from Coordinated Universal Time (UTC) as maintained by the National Metrology Institute of the Russian Federation UTC(SU) as part of the State Time and Frequency Service, and almanacs (approximate ephemerides) of all other GLONASS satellites. Note that, unlike GPS System Time, for example, GLONASS System Time has no integer offset from UTC and so leap-second jumps are added to GLONASS System Time simultaneously with those added to UTC. Note, however, that GLONASS System Time is offset by a constant three hours to match Moscow Standard Time (MSK, an abbreviation for Moscow).
The full message lasts 2.5 minutes, and is continuously repeated between ephemeris updates (nominally once every 30 minutes), but the ephemeris and clock information is repeated every 30 seconds.
The GLONASS authorities have not published, at least publicly, details of the VT-code navigation message. It is known, however, that the full message takes 12 minutes and that the ephemeris and clock information are repeated every 10 seconds.
Geodetic System. GLONASS ephemerides are referenced to the Parametry Zemli 1990 (PZ-90 or, in English translation, Parameters of the Earth 1990, PE-90) geodetic system. PZ-90 replaced the Soviet Geodetic System 1985, SGS 85, used by GLONASS until 1993. PZ-90 is a terrestrial reference system with its coordinate frame defined in the same way as that of the International Terrestrial Reference Frame (ITRF). The initial realization of PZ-90 had an accuracy of one or two meters.
However, in an effort to bring the system closer to the ITRF (and GPS’s WGS 84 geodetic reference system), two updates to PZ-90 were carried out. The first update, resulting in PZ-90.02 (referring to 2002), was adopted for GLONASS operations on Sept. 20, 2007, and brought the frame of the broadcast orbits (and hence derived receiver coordinates) closer to ITRF and WGS 84. Another realization, PZ-90.11, adopted on Dec. 31, 2013, reportedly reduced the differences to the sub-centimeter level.
TABLE 1 lists the defining constants and parameters of PZ-90.
TABLE 1. Fundamental geodetic constants and some of the parameters of the PZ-90 geodetic system as used by GLONASS.
The new GLONASS-K satellites transmit additional signals. GLONASS-K1 transmit a CDMA signal on a new L3 frequency (1202.025 MHz), and GLONASS-K2, in addition, will feature CDMA signals on the L1 and L2 frequencies.
FIGURE 3. Circular reflector array on a GLONASS-K1 satellite, surrounding navigation signal inner antenna elements. Photo from Reshetnev Information Satellite Systems.
Control Segment. Similar to GPS and other GNSSs, GLONASS requires a network of ground stations for monitoring and maintaining the satellite constellation and for determining the orbits of the satellites and behavior of their operating AFSs. The tracking network uses stations only within the territory of the former Soviet Union, supplemented with satellite laser ranging stations to help with orbit determination since all GLONASS satellites contain laser reflectors (see FIGURE 3).
Having a non-global network of tracking stations for determining the satellite orbits and AFS behavior results in slightly degraded GLONASS signal-in-space range error (SISRE). Recently, a number of tracking stations overseas have been established in conjunction with the development of the Russian satellite-based augmentation system (SBAS), the System for Differential Correction and Monitoring (SDCM). SDCM will function in a similar fashion to the Wide Area Augmentation System or WAAS, the U.S. SBAS, and the other SBASs in operation. The addition to the tracking network of the overseas SDCM stations, which already includes stations in Antarctica and South America with more stations coming, could help improve SISRE. Roscosmos also uses a global network of IGS and other tracking stations to monitor the health of the GLONASS constellation (see FIGURE 4).
FIGURE 4. Roscosmos global GLONASS satellite health monitoring network with 22 reporting stations on Oct. 18, 2017, between 13:00 and 14:00 MSK.
Performance. SISRE has improved over the years and is currently at the level of about 1 to 2 meters. In part, this is due to the better performance of the on-board AFSs carried by the latest GLONASS-M satellites compared to the first GLONASS-M satellites. Their relative one-day stability has improved from 10-13 to 2.4 × 10-14. FIGURE 5 shows a time series of recent values of SISRE determined by the Information and Analysis Center for Positioning, Navigation and Timing. These error levels can result in pseudorange-based positioning errors using GLONASS broadcast orbits and clocks about a factor of two worse than those provided by GPS — although, at any given instant, positioning accuracy will also be impacted by atmospheric effects and multipath and these could dominate the signal-in-space errors.
FIGURE 5. GLONASS daily root-mean-square signal-in-space range error in meters as determined by the Information and Analysis Center for Positioning, Navigation and Timing.
Much higher positioning accuracies can be obtained using GLONASS orbits and clocks provided by the IGS and its participating analysis centers. This is particularly true if carrier-phase measurements are used instead of or as a supplement to pseudorange measurements. A combination of appropriately weighted GPS and GLONASS measurements has shown to be beneficial in terms of availability, accuracy and efficiency, especially for high-accuracy positioning carried out using the real-time kinematic or RTK approach. Furthermore, the precise point positioning (PPP) technique, based on real-time or post-processing of dual-frequency carrier-phase measurements with precise satellite ephemeris and clock data, has demonstrated that kinematic decimeter-level accuracy is possible using GLONASS data or GLONASS data in combination with GPS data. GLONASS-only static PPP solutions over 24 hours have achieved accuracies at the millimeter level.
Users. The initial uptake of GLONASS by both civil and military users in the former Soviet Union and subsequently in Russia, not to mention outside Russia, was minimal. Prototype GLONASS-only receivers were developed for the military, and foreign GPS/GLONASS receivers were developed by several manufacturers for scientific and other advanced applications. The IGS added a set of GLONASS-tracking receivers to its network in 1998 and has continuously increased the number of such receivers since then. However, consumer use of GLONASS both within and outside Russia has only recently taken off with the development of GLONASS-only and combined GPS/GLONASS chipsets. Such chipsets are now featured in many mobile phones and in handheld GNSS receiver and vehicle navigation units.
NEW AND IMPROVED
As previously mentioned, the GLONASS-K1 satellites include a CDMA signal accompanying the legacy FDMA signals on a new L3 frequency of 1202.025 MHz. The ranging-code chipping rate for the CDMA signal is 10.23 megachips per second with a period of 1 milliseconds. It is modulated onto the carrier using quadrature phase-shift keying (QPSK), with an in-phase data channel and a quadrature pilot channel. The set of possible ranging codes consists of 31 truncated Kasami sequences. (Kasami sequences, introduced by Tadao Kasami, a noted Japanese information theorist, are binary sequences of length 2m – 1 where m is an even integer. These sequences have good cross-correlation values approaching a theoretical lower bound. The Gold codes used in GPS are a special case of Kasami codes.) The full length of these sequences is 214 – 1 = 16,383 symbols, but the ranging code is truncated to a length of N = 10,230 with a period of 1 milliseconds.
The associated navigation message symbols are transmitted at a rate of 100 bits per second with half-rate convolution coding. The so-called navigation message superframe (2 minutes long) will consist of 8 navigation frames (NFs) for 24 regular satellites in the GLONASS first modernization stage and 10 NFs (lasting 2.5 minutes) for 30 satellites in the future. Each NF (15 seconds long) includes 5 strings (3 seconds each). Every NF has a full set of ephemerides for the current satellite and part of the system almanac for three satellites. The full system almanac is broadcast in one superframe.
The lighter, unpressurized K1 satellites feature two cesium and two rubidium AFSs. The relative daily stability of one of the rubidium AFSs on a K1 satellite is reported to be 4 ×10-14. As a result, the SISRE for this satellite is about 1 meter. Plans call for adding a CDMA signal to L2 on future versions of the K1 satellites, dubbed K1+ (see below).
GLONASS-K2 Satellites. These satellites will be heavier than the K1 and K1+ satellites with greater capabilities including a CDMA signal at the GPS/Galileo L1/E1 frequency. Reshetnev ISS will initially build two K2 satellites before going into mass production. It had been planned to transition to the K2 satellites much sooner, only launching the two K1 satellites now in orbit. But apparently plans changed because of the sanctions restricting the delivery of radiation-resistant electronic components from the West.
Now, Reshetnev ISS will build an additional nine GLONASS-K1 satellites. It’s not clear how many of these might be of the K1+ variety. The GLONASS-K1 satellites will now be transition satellites between the existing GLONASS-M satellites (including the half-dozen or so that have been manufactured and stored on the ground for future launch as needed) and the future GLONASS-K2 satellites.
One of the first K2 satellites will host a passive hydrogen maser (PHM) AFS. The PHM has been under development for about a decade, and multiyear ground tests displayed a reliability and one-day stability of 5 × 10-15. It is expected to contribute to future 0.3-meter SISRE.
According to a recent report, GLONASS-K2 satellites will begin flight tests in 2018, with mass production of GLONASS-K2 satellites to begin in the 2019–2020 time frame.
Improved Tracking Networks. The development of the SDCM and its associated tracking network has already been mentioned. The SDCM network stations are equipped with combined GPS/GLONASS dual-frequency receivers, hydrogen maser atomic clocks and direct communication links for real-time data transfer. As mentioned earlier, GLONASS authorities are looking at whether additionally using the SDCM stations for GLONASS orbit and clock determination would significantly enhance the accuracy of the broadcast data.
CONCLUSION
GPS, the oldest GNSS, is continuing to modernize and will soon launch the first Block III or GPS III satellite. Already, GPS Block IIR-M and Block IIF satellites are transmitting new signals. Galileo is fielding modern satellites right from the get go, and BeiDou is about to start launching the operational version of its BeiDou-3 satellites. GLONASS is not to be outdone. It has provided useful positioning, navigation and timing services since at least 1996. While at times the service level has dropped below acceptable levels, it is now a dependable system and, with announced improvements, will be a contender in the future world of multi-GNSS.
FURTHER READING
Official GLONASS Update
“GLONASS Programme Update” by I. Revnivykh presented at the 11th Meeting of the International Committee on Global Navigation Satellite Systems, Sochi, Russia, Nov. 6–11, 2016.
In-depth Description of GLONASS
“GLONASS” by S. Revnivykh, A. Bolkunov, A. Serdyukov and O. Montenbruck, Chapter 8 in Springer Handbook of Global Navigation Satellite Systems, edited by P.J.G. Teunissen and O. Montenbruck, published by Springer International Publishing AG, Cham, Switzerland, 2017.
“GLONASS: Developing Strategies for the Future” by Y. Urlichich, V. Subbotin, G. Stupak, V. Dvorkin, A. Povalyaev and S. Karutin in GPS World, Vol. 22, No. 4, April 2011, pp. 42–49.
“GLONASS: Review and Update” by R.B. Langley in GPS World, Vol. 8, No. 7, July 1997, pp. 46–50. Correction: GPS World, Vol. 8, No. 9, Sept. 1997, p. 71. Available on line:
“GLONASS Spacecraft” by N.L. Johnson in GPS World, Vol. 5, No 11, Nov. 1994, pp. 51–58.
At Intergeo 2017, Septentrio debuted the Altus NR3: a multi-frequency, quad-constellation (GPS, GLONASS, BeiDou and Galileo) RTK receiver for survey and GIS applications.
The Altus NR3 features Septentrio’s AIM+ interference mitigation and monitoring system, allowing continued operation in the presence of both intentional and non-intentional interference. According to the company, it combines advanced GNSS features with a robust communications suite in a compact, low-power and easy-to-use unit.
The Altus NR3 is configurable as either a rover or a base station. It offers one-touch logging and Septentrio’s on-board web interface so users can monitor and configure the unit as well as collect data using any Wi-Fi-capable device.
Data collection is done using either SurvCE or Septentrio’s PinPoint Data Collector with data updating to the cloud. Septentrio’s open interface and fully documented data formats are widely supported, making the Altus NR3 easy to integrate into any existing workflow, the company said.
“We’ve built on the flexibility, reliability and ease-of-use that our Altus line is famous for, and we’ve added all-in-view RTK and the most the most advanced interference mitigation system on the market today,” said Gustavo Lopez, product manager at Septentrio. “Locations with bad visibility or at risk of interference that were previously off limits can now benefit from high-precision GNSS positioning, saving both time and cost.”
Tersus GNSS Inc. has released a major upgrade to its Precis-BX306 RTK board with new and improved GPS/GLONASS functionality. Tersus GNSS is a manufacturer of high-precision GNSS real-time kinematic (RTK) boards, receivers and systems.
Precis-BX306 Board Easy Kit.
The new version of Precis-BX306 supports up to 20-Hz RTK solution and raw measurement output, which can be integrated with autopilots and inertial navigation units.
With improved algorithms, the new Precis-BX306 demonstrates its ability that the 30-km baseline can be fixed quickly, the company said.
Moreover, the dynamic fix rate shows an advantage comparing to the majority of competitive solutions. Stable fix rate is achieved when it is working under city valley, tree, and other challenging environment.
“The technology changes made in this version give a whole new user experience for our customers,” said Xiaohua Wen, founder and CEO. “With the updated capabilities, the Precis-BX306 is particularly useful for drones, surveyors and geographic information system data users interested in an affordable RTK solution.”
This latest version of Precis-BX306 is pin-to-pin compatible with major GNSS boards in the market, offering a flexible interface. Event mark and PPS are supported as always. All enhanced features demonstrate Tersus’ commitment to the needs of customers who value dynamic accuracy and stability.
The new Precis-BX306 is available for order and delivery immediately, and it will be shown at Intergeo in Berlin, Germany, Sept. 26-28.
The second satellite of Japan’s Quasi-Zenith Satellite System (QZSS) has started transmitting navigation signals. QZS-2, or Michibiki-2, was launched on June 1, 2017, and joins its predecessor QZS-1 (Michibiki-1), which has been in orbit since September 2010.
Both satellites have been placed into inclined geosynchronous, elliptical orbits, which enable extended satellite visibility periods over Japan and are characteristic features for this regional navigation system.
The third satellite QZS-3 was launched on Aug. 19, 2017, into a geostationary orbit. If all goes according to plan, a fourth satellite in an eccentric orbit will follow by the end of this year and complete the constellation.
Three of six new Lockheed Martin-developed receivers are now deployed at U.S. Air Force monitoring stations to maintain the accuracy of GPS satellite signals.
In June, the first Monitor Station Technology Improvement Capability (MSTIC) receiver became operational at Cape Canaveral Air Force Station, Florida. Upgrades continued at USAF monitoring stations at Kwajalein Atoll and Hawaii. These upgrades from early 1990s technology are part of an overall effort to modernize the current GPS ground control system, known as the Architecture Evolution Plan Operational Control Segment.
MSTIC software-defined radio technology replaces legacy receivers’ hardware-based application-specific integrated circuit platform. MSTIC leverages commercial off-the-shelf hardware without the need for custom firmware. Standard interfaces and architecture configurability simplify sustainment and enable MSTIC software to migrate to new hardware platforms as commercial vendors increase processing power, improve reliability and enhance cybersecurity. MSTIC enables remote application of mission-specific software updates to improve performance and enable reception of modernized GPS signals, according to the company.
The three remaining GPS Monitoring Stations will be upgraded with MSTIC receivers by the end of 2017.
The navigation payload before integration into the second GPS III SV, which now is in environmental testing. (Photo: Harris)
GPS III Payloads Delivered
Harris Corporation has delivered the third of 10 advanced navigation payloads to Lockheed Martin. The payloads will increase accuracy, signal power and jamming resistance for GPS III satellites. They feature a Mission Data Unit (MDU) with a 70-percent digital design that links atomic clocks, radiation-hardened computers and powerful transmitters, enabling signals three times more accurate than those on current GPS satellites. The new payloads also boost satellite signal power, increase jamming resistance by eight times and help extend the satellite’s lifespan.
The payload was integrated into GPS III SV03 over the summer. The first navigation payload is integrated aboard GPS III SV01, which is in storage awaiting expected 2018 launch.
Harris announced it is in full production and on target to deliver the fourth GPS III navigation payload to Lockheed Martin in fall. Harris is also developing a fully digital MDU for the U.S. Air Force’s GPS III Space Vehicles 11+ acquisition. The new MDU will be demonstrated in fall 2017 and provides even greater flexibility, affordability and accuracy versus existing GPS satellites.
Next GLONASS-M Readied
The Russian navigation satellite GLONASS-M 52 moved from ISS-Reshetnev Company’s assembly plant to the Plesetsk Cosmodrome launch site about 800 km north of Moscow in August. One of the system’s ground spares, it was built more than two years ago and stored awaiting launch. The satellite is due to launch in September.
There are six GLONASS-M satellites in ground reserve.
The Russian navigation satellite GLONASS-M 52 has traveled from ISS-Reshetnev Company’s facilities in Zheleznogorsk to the Plesetsk launch site, reported ISS-Reshnetev on Aug. 25.
GLONASS-M 52 is one of the GLONASS system’s ground spares. It was built by ISS-Reshetnev Company more than two years ago and was stored at the company’s facilities waiting for launch.
Before flying to the cosmodrome, GLONASS-M 52 was thoroughly tested and prepared for transportation. ISS-Reshetnev technicians used multi-layer insulation and special cases to protect the satellite’s sensitive equipment from damage during transport.
GLONASS-M 52 was loaded in a special container and flown to the Plesetsk cosmodrome on an IL-76 aircraft accompanied by ISS-Reshetnev specialists. The satellite is due to launch in September.
There are currently six GLONASS-M satellites in the ground reserve of the GLONASS navigation satellite system. All of them were constructed by ISS-Reshetnev Company in strict compliance with contract terms and are now stored at its facilities.
Each of these satellites is on standby for launch and can be shipped to the cosmodrome whenever needed to augment the GLONASS orbital constellation.
GLONASS-M 52 will replace a retired long-lived satellite that carried out its mission in orbit 1.5 times longer than initially designed.
GLONASS-M 52 is expected to launch in September. (Photo: ISS-Reshetnev)
Figure 1. Galileo constellation and occupation status of orbital slots (RAAN: right ascension of the ascending node, May 9, 2017). (Source: ESA)
What to Expect with the Current Constellation
This article demonstrates the benefits of Galileo integration for high-precision real-time kinematic (RTK) through representative case studies, considering baseline length, multipath impact and tree canopy.
The results confirm usability of the current Galileo constellation in high-precision RTK applications and show improved availability, accuracy, reliability and time-to-fix in difficult measuring environments.
Plus, Galileo-only RTK positions are compared with GPS-only and GLONASS-only solutions.
By Xiaoguang Luo, Jun Chen and Bernhard Richter, Leica Geosystems AG
Until now, based on simulated and observed data, the benefits of Galileo (FIGURE 1) for high-precision RTK have been investigated in single-base RTK and network RTK solutions. Building on the results of previous studies that frequently employed theoretic analysis and simulation, we present the benefits of Galileo for high-precision RTK based on real observations from the current Initial Operational Capability (IOC) satellite constellation. Using up-to-date real-time corrections including Galileo, we analyze the performance of network RTK under different measuring conditions with respect to availability, accuracy, reliability and time-to-fix.
To achieve the maximum inter-operability with other GNSS con-stellations, all the Galileo signals in the E1 and E5 band, i.e. E1, E5a, E5b and AltBOC (alternative binary offset carrier), are used for positioning in the latest proprietary firmware and receivers (see “Manufacturers” section for details).
The Galileo E1 signal is overlapped with the GPS L1 signal at a center frequency of 1575.420 MHz, whereas the Galileo E5a and GPS L5 signals are overlapped at 1176.450 MHz. As far as BeiDou is concerned, the E5b frequency of Galileo corresponds to the B2 frequency of BeiDou-2 at 1207.140 MHz.
The AltBOC signal is also supported in order to benefit from its superior performance in multipath suppression. The availability of more than two frequencies is beneficial for ionospheric modeling, which plays an important role in ambiguity resolution on the fly.
In addition, multi-frequency RTK provides more immunity to temporary interruption of GNSS signals caused by interference or by site-specific effects like multipath. When forming linear combinations, the incorporation of multi-frequency signals enhances flexibility and robustness, where the mathematical correlations introduced by including the same signal in different linear combinations of the same type need to be handled properly in RTK algorithms.
By enabling the tracking of Galileo satellites in the aforementioned firmware, the Galileo signals will be used in different RTK position types by default, including navigation position, phase-aided differential code position, extended RTK (xRTK) position and RTK fixed position. When compared to a standard RTK fix, an xRTK fix is provided at a slightly lower accuracy level, but with higher availability in difficult environments such as urban canyons and dense canopy.
In terms of RTK correction data formats, Galileo is included in the standardized RTCM v3 MSM format and in the proprietary 4G format. To use Galileo in network RTK, the real-time products provided by network correction services need to include Galileo as well. In the latest version of a proprietary GNSS network software, Galileo is used in network processing to provide RTK corrections via the individualized master-auxiliary (iMAX) method and the virtual reference station (VRS) method in the RTCM 3.2 MSM formats.
RTK PERFORMANCE CHARACTERISTICS
Multi-constellation and multi-frequency GNSS RTK is a complex real-time process, aiming to provide cm-level positioning accuracy with as few as possible data epochs for variable user kinematics and even in difficult measuring environments. Therefore, RTK performance characteristics need to be carefully selected to be able to evaluate the system as a whole and to address users’ concerns in their applications.
The following parameters are used in this article to assess the benefits of Galileo for high-precision RTK:
Satellite usage. Number of satellites used in RTK fixed solutions with an elevation cut-off angle of 10°;
Availability. Percentage of RTK fixed positions relative to all positions obtained during a time period;
Accuracy. Deviation of RTK fixed positions from ground truth with a higher degree of accuracy, where the ground truth can be determined by means of a total station or by post-processing long-term GNSS data;
Reliability. Percentage that the position error (with respect to ground truth) is less than 3 x coordinate quality (CQ) indicator;
Time to Fix. Time needed to regain an RTK fixed solution after losing ambiguity fix provided that GNSS signal tracking is not interrupted.
OPEN-SKY CASE STUDY
The open-sky case study was performed in the Heerbrugg testbed. Two receivers were connected to a single antenna via a four-way antenna splitter. One receiver received four-system iMAX corrections in the RTCM v3 MSM format over a short baseline of 2 km, whereas the other received RTK data of the same type over a long baseline of 116 km. By considering different baseline lengths, the open-sky experiment focused on the usability of the current Galileo constellation in GNSS RTK under normal conditions. Two days of 1-Hz GNSS data were investigated with respect to satellite usage and positioning accuracy.
Using different combinations of GNSS to analyze the short baseline data — GPS+GLO (GG), GPS+GLO+BDS (GGB) and GPS+GLO+GAL+BDS (GGGB) — the mean numbers of used satellites are 15, 17 and 20, respectively, where the elevation cut-off angle was set to 10°. On average, three Galileo satellites contribute to RTK fixed solutions.
For the four-system combination GGGB, Figure 2 shows the satellite usage for each individual system over the two-day period. It can be seen that for a short baseline of 2 km, a maximum number of four Galileo satellites can be used for positioning. In fact, during 80.3% of the whole test period, the number of Galileo satellites used in RTK fixed solutions is equal to or greater than the number of BeiDou satellites used.
Figure 2. Number of satellites used in RTK fixed positions with GGGB under open sky (iMAX, RTCM v3 MSM, baseline length: 2 km, GGGB: GPS+GLO+GAL+BDS, DOY: day of year).
Table 1 provides statistics on Galileo satellite usage in case of GGGB for different baseline lengths. As would be expected, the number of Galileo satellites used decreases with an increasing baseline length. In approximately 41% of the cases, three Galileo satellites are used in the short baseline test, whereas two Galileo satellites are used in the long baseline test.
Moreover, the probability that no Galileo satellites are involved in a four-system combined solution grows significantly from 1.9% to 15.0% as the baseline length increases from 2 km to 116 km. The probability that only one Galileo satellite is used under open sky is relatively small, amounting to around 0.5%. This is reasonable since no benefits for high-precision RTK are expected in this particular situation. Regarding the short baseline case, there is a 97.7% probability that at least two Galileo satellites are used for positioning, whereas this probability decreases to 84.4% in the long baseline case.
Table 1. Probability [%] that n Galileo satellites are used in RTK fixed positions with GGGB during the two-day period of the open-sky experiment (iMAX, RTCM v3 MSM, GGGB: GPS+GLO+GAL+BDS).In terms of positioning accuracy, Figure 3 compares the 3D errors from analyzing the long baseline data with different GNSS constellations. Regarding the entire two-day period illustrated in Figure 3a, the integration of BeiDou (GG vs. GGB) and Galileo (GGB vs. GGGB) results in higher position repeatability with more consistent errors. For a selected period of 12 hours, Figure 3b highlights the advantages of Galileo in reducing large 3D errors from 6–8 cm to 3–4 cm, where two or three Galileo satellites are used in case of GGGB.
Figure 3. 3D errors of RTK fixed positions under open sky (iMAX, RTCM v3 MSM, baseline length: 116 km, GG: GPS+GLO in green, GGB: GPS+GLO+BDS in blue, GGGB: GPS+GLO+GAL+BDS in red, DOY: day of year) (a) Entire two-day period, (b) Selected 12-hour period (28–40 h).
MULTIPATH CASE STUDY
In this case study, a GNSS smart antenna was set up in a location with strong multipath effects, where GNSS signals were obstructed and reflected by the surrounding buildings (Figure 4). This test setup simulates the use case that a user measures a point near a building with degraded GNSS signal reception, even at high elevation angels.
Figure 4. Test setup in a strong multipath environment in Heerbrugg (rover: GS16, antenna height: 1.8 m) (a) View from the south, (b) View from the north.
The default elevation cut-off angle of 10° was applied. The receiver received four-system VRS corrections in the RTCM v3 MSM format, where the distance to the physical reference station was approximately 200 m. Three hours of 1-Hz GNSS data were analyzed with respect to accuracy, reliability and time to fix.
Figure 5 illustrates the 3D errors from multi-GNSS RTK with and without Galileo (GGGB vs. GGB), along with the number of used satellites. Regarding the periods marked with dashed rectangles, the inclusion of two or three Galileo satellites (Figure 5b) leads to significant improvements in positioning accuracy at the few cm to dm level (Figure 5a). By comparing the empirical cumulative distribution function (CDF) of the 3D errors, the probability that 3D error is within 5 cm increases from 70% to 85% if Galileo is used, even with a maximum number of three satellites.
Figure 5. Impact of Galileo integration on RTK positioning accuracy under strong multipath (VRS, RTCM v3 MSM, GGB: GPS+GLO+BDS in blue, GGGB: GPS+GLO+GAL+BDS in red, DOY: day of year) (a) 3D errors of RTK fixed positions, (b) Number of used satellites (Galileo in green).
Tables 2 and 3 provide the root mean square (RMS) errors and reliability of RTK fixed positions from the multipath experiment, respectively. By using Galileo in high-precision RTK, the 3D RMS error is significantly reduced by 56.3% in this case study, from 0.080 m (GGB) to 0.035 m (GGGB). When compared to the horizontal components, the height RMS error shows a larger relative improvement of 58.7% due to Galileo integration. The reliability reflects the consistency between the actual position error with respect to ground truth and the CQ indicator estimated based on mathematical models in RTK algorithms. As shown in Table 3, the 3D reliability improves by 7.3%, from 88.2% (GGB) to 95.5% (GGGB), where the increases for the horizontal components and height are comparable.
Table 2. Root mean square errors [m] of RTK fixed positions under strong multipath (VRS, RTCM v3 MSM, GGB: GPS+GLO+BDS, GGGB: GPS+GLO+GAL+BDS).Table 3. Reliability [%] of RTK fixed positions under strong multipath (VRS, RTCM v3 MSM, GGB: GPS+GLO+BDS, GGGB: GPS+GLO+GAL+BDS).The time to fix (TTF) was determined by constantly re-initializing RTK once an ambiguity fix was gained. During the whole period of repeatedly resetting the RTK filter, the GNSS signals were tracked continuously without interruption. A total of 765 TTF values were obtained with GGB, whereas 1,128 TTF estimates were available with GGGB. The significantly larger number of the TTF samples from GGGB indicates higher availability of RTK fix if Galileo is used.
Figure 6 shows the statistical distribution of TTF with respect to Galileo integration. As can be seen in the empirical CDF in Figure 6a, it takes shorter time for GGGB to regain an ambiguity fix. As an example, GGGB allows ambiguity resolution within 5 s (10 s) with 46% (87%) probability, which is 29% (16%) higher than GGB. Regarding the boxplots of TTF in Figure 6b, GGGB shows a smaller median (by 25% from 8 s to 6 s) and a smaller interquartile range (IQR; by 50% from 4 s to 2 s) than GGB, where the IQR is the length of the box. This indicates that the integration of Galileo enables a faster ambiguity resolution with more consistent fixing performance.
Figure 6. Impact of Galileo integration on time to fix (TTF) statistics under strong multipath (VRS, RTCM v3 MSM) (a) Empirical cumulative distribution function (CDF) of TTF, (b) Boxplot of TTF with median and interquartile range (IQR).
CANOPY CASE STUDY
In this case study, a receiver was connected to an antenna under tree canopy (Figure 7), where GNSS signals are blocked, attenuated and reflected, leading to decreased number of observations, low data quality and degraded RTK performance.
Under these circumstances, the inclusion of Galileo satellites transmitting multi-frequency signals could be particularly beneficial for high-precision RTK. Using an elevation cut-off angle of 10°, the receiver received four-system iMAX corrections in the RTCM v3 MSM format, where the baseline length was 116 km. A long baseline was intentionally selected as an additional challenge for the RTK system. About seven hours of 1-Hz GNSS data were investigated regarding availability, accuracy and reliability.
Figure 7. Test setup under canopy in Heerbrugg (rover: GS10, antenna: AS10).
Figure 8 illustrates the impact of Galileo integration on RTK availability and accuracy under canopy, along with the number of used satellites. As can be seen in Figure 8a, the inclusion of Galileo improves the availability of RTK fixed positions by 12.2%, from 65.7% (GGB) to 77.9% (GGGB). Moreover, dm-level position errors are largely reduced, as shown in FigURE 8c. The improvements in availability and accuracy are achieved by using up to three Galileo satellites (Figure 8b). This demonstrates that the current Galileo constellation in the IOC phase brings considerable benefits to high-precision RTK under canopy conditions.
Figure 8. Impact of Galileo integration on RTK availability and accuracy under canopy (iMAX, RTCM v3 MSM, baseline length: 116 km, GGB: GPS+GLO+BDS in blue, GGGB: GPS+GLO+GAL+BDS in red, DOY: day of year) (a) Availability of RTK fixed positions over time, (b) Number of used satellites (Galileo in green), (c) 3D errors of RTK fixed positions.
Tables 4 and 5 provide the RMS errors and reliability of RTK fixed positions from the canopy experiment, respectively. The main factors degrading the RTK accuracy in this case study are not only the canopy environment, but also the long baseline length of 116 km. It can be seen in Table 4 that the integration of Galileo leads to a significant reduction of 3D RMS error by 23.7%, from 0.114 m (GGB) to 0.087 m (GGGB).
By comparing the 2D and 1D RMS errors, the benefits of Galileo for the height are more dominant than for the horizontal components, which was also observed in the multipath experiment (Table 2). In terms of reliability, only slight (below 2%) increases are visible in Table 5. 116km baseline length and heavy canopy are considered extreme conditions and beyond the standard conditions relevant for specifications. Considering reliability together with availability (Figure 8a), it is encouraging to see that both the RTK performance characteristics are improved in this case study.
Table 4. Root mean square errors [m] of RTK fixed positions under canopy (iMAX, RTCM v3 MSM, baseline length: 116 km, GGB: GPS+GLO+BDS, GGGB: GPS+GLO+GAL+BDS).Table 5. Reliability [%] of RTK fixed positions under canopy (iMAX, RTCM v3 MSM, baseline length: 116 km, GGB: GPS+GLO+BDS, GGGB: GPS+GLO+GAL+BDS).
GALILEO-ONLY RTK
To optimize the performance of multi-GNSS RTK positioning, the individual systems need to be fully understood and mastered. With a previous firmware release in August 2014, mass-market devices were able to perform GLONASS-only and BeiDou-only high-precision RTK. In 2014 tests, we compared the performance of GPS-only, GLONASS-only and BeiDou-only RTK at different accuracy levels. Considering that Galileo has reached the IOC phase, it is reasonable to assess the Galileo-only RTK performance with the latest firmware.
Due to the limited number of usable Galileo satellites, Galileo-only RTK positioning was carried out in the Heerbrugg open-sky testbed over a very short baseline of 1 m. In addition, the elevation cut-off angle was set to 0° in order to track as many Galileo satellites as possible simultaneously. Two receivers were connected to two choke-ring antennas with good low-elevation tracking ability. Single-base RTK positioning was performed with four-system corrections in the RTCM v3 MSM format. About one hour of 1-Hz GNSS data was analyzed with a special focus on positioning accuracy.
Figure 9 shows the 3D errors from GPS-only, GLONASS-only and Galileo-only RTK positioning, where the numbers of used satellites are 8–11, 7–9 and 5–6, respectively. During the test period, only three or four BeiDou satellites were tracked with poor geometry, making BeiDou-only RTK impossible. As the figure shows, the 3D errors from GPS-only and Galileo-only RTK are at a comparable level with similar RMS values, whereas the 3D RMS error from GLONASS-only RTK is almost twice as large as the GPS/Galileo-only case. Note that when compared to GPS-only RTK, almost half as many satellites are used in Galileo-only RTK.
Figure 9. 3D errors of RTK fixed positions from GPS-only, GLONASS-only and Galileo-only RTK under open sky (single-base RTK, baseline length: 1 m, RTCM v3 MSM, DOY: day of year, RMS: root mean square).
Figure 10 displays the statistical distribution of the 3D errors from GPS-only, GLONASS-only and Galileo-only RTK positioning. Regarding the empirical CDF in Figure 10a, GPS/Galileo-only RTK shows a clearly more favorable error distribution than the GLONASS-only case. Using only GPS or Galileo, the probability that 3D error is within 1 cm is above 80%, which is approximately 30% higher than using only GLONASS. For 3D errors ranging between 5 mm and 1.7 cm, Galileo-only RTK even provides a slightly higher cumulative probability than the GPS-only case. The 3D error boxplots in Figure 10b illustrate a similar pattern between GPS-only and Galileo-only RTK, which is superior to GLONASS-only RTK due to the significantly smaller median and IQR.
Figure 10. 3D error statistics from GPS-only, GLONASS-only and Galileo-only RTK under open sky (single-base RTK, baseline length: 1 m, RTCM v3 MSM). (a) Empirical cumulative distribution function (CDF) of 3D errors, (b) Boxplot of 3D errors (IQR: interquartile range).
CONCLUSIONS
With the declaration of Galileo Initial Services in December 2016, for the first time ever all GNSS users worldwide are able to use the positioning, navigation and timing information provided by Galileo’s global satellite constellation. Upon full system completion by 2020, Galileo will play an important role in high-precision GNSS applications for users around the world. This article showed representative case studies to understand the benefits of the current Galileo constellation for high-precision RTK. In addition to a multi-GNSS solution, the performance of Galileo-only RTK was presented. The main findings from the case studies can be summarized as follows:
In the open-sky test, with an elevation cut-off angle of 10°, on average three Galileo satellites can be used for high-precision multi-GNSS RTK. This leads to cm-level improvements in coordinate repeatability over a long baseline of 116 km.
In the multipath case study, the additional use of two or three Galileo satellites produces significant enhancements in positioning accuracy at the few cm to dm level, where the benefits for the height component are more significant. Moreover, the integration of Galileo increases the 3D reliability of RTK fixed positions by 7.3% and reduces the median time to fix by 2 s (25%).
In the canopy experiment, the inclusion of Galileo improves the availability of RTK fixed solutions by 12.2%. Furthermore, dm-level position errors are largely reduced.
When compared to GPS-only RTK, Galileo-only RTK provides a similar positioning accuracy over a 1-m baseline under open sky, where almost half as many satellites are used. The 3D RMS error from GLONASS-only RTK is approximately twice as large as the GPS/Galileo-only case.
The promising results achieved through Galileo integration already indicate the very important role of the European GNSS in high-precision, multi-frequency and multi-constellation RTK positioning. During the deployment of the Galileo system, more benefits can be expected in the near future.
ACKNOWLEDGMENTS
The staffs of Leica Geosystems AG (Heerbrugg/Switzerland), Christian Waese and Youssef Tawk, are gratefully acknowledged for support in setting up the variety of RTK network streams.
MANUFACTURERS
SmartWorx 6.16 of Leica Viva GNSS is the latest firmware cited and used in these high-precision RTK tests. Leica GNSS Spider 7.0.0 furnished the GNSS real-time corrections. The open-sky case study used two Leica Viva GS10 units connected to a Leica Viva AS10 antenna via a four-way antenna splitter. The multipath case study used a Leica Viva GS16 GNSS smart antenna. The canopy case study used a Leica Viva GS10 receiver and a Leica Viva AS10 antenna. The Galileo-only RTK test used two Leica Viva GS10 receivers and two Leica AR25 choke ring antennas.
Dana Goward President, Resilient Navigation and Timing (RNT) Foundation
An apparent mass GPS spoofing attack in June involved more than 20 vessels in the Black Sea and suggests that Russia may be aggressively experimenting with signal disruption and spurious substitution.
“GPS equipment unable to obtain GPS signal intermittently since nearing coast of Novorossiysk, Russia. Now displays HDOP 0.8 accuracy within 100m, but given location is actually 25 nautical miles off…”
Subsequent dialog with the ship’s master and examination of various documents and screen grabs he furnished enabled navigation experts to conclude this was a fairly clear case of spoofing: sending false signals to cause a receiver to provide false information. Other vessels in the vicinity experienced the same problem.
The RNT Foundation has received numerous anecdotal reports of maritime problems with the automatic identification system (AIS), a tracking system used for collision avoidance on ships, and with GPS in Russian waters, though this is the first well-documented public account.
Russia has very advanced capabilities to disrupt GPS. More than 250,000 cell towers in Russia have been equipped with GPS jamming devices as a defense against attack by U.S. missiles. And there have been press reports of Russian GPS jamming in both Moscow and the Ukraine. In fact, Russia has boasted that its capabilities “make aircraft carriers useless.”
The U.S. director of National Intelligence issued a report on May 11 that states that Russia and other actors are focusing on improving their capability to jam U.S. satellite systems.
Assuming Russia is behind this, why would they do such a thing? Possibly to encourage use of GLONASS or their terrestrial loran system, Chayka, instead of GPS. Possibly for some security reason known only to them.
Whatever the reason, it reminds us of the vulnerability of GPS signals, and of the plethora of motives that “bad actors” — governmental or private criminal interests — may have to disrupt and deceive GNSS users.
And of the U.S. Coast Guard’s advice about GPS and all satnav: “Trust But Verify.”
Dana Goward is president of the Resilient Navigation and Timing Foundation. He is the proprietor at Maritime Governance LLC. In August 2013, he retired from the federal Senior Executive Service, having served as the maritime navigation authority for the United States. As director of Marine Transportation Systems for the U.S. Coast Guard, he led 12 different navigation-related business lines budgeted at more than $1.3 billion per year. He has represented the U.S. at IMO, IALA, the UN anti-piracy working group and other international forums. A licensed helicopter and fixed-wing pilot, he has also served as a navigator at sea and is a retired Coast Guard Captain.
In this article, author Urs Hugentobler looks at the history of laser ranging to navigation satellites, how that ranging has improved the accuracy of the orbits of those satellites and what the future portends for this important contribution to space geodesy.
INNOVATION INSIGHTS with Richard Langley
THE LASER. It might not be in the top 10 of the most important inventions of all time, but Time magazine rated it among the most important developments of the 20th century, listing it fifth after the automobile, the radio, the television and the transistor. Lasers are now ubiquitous: they scan our purchases at the supermarket checkout; they let us read and write data on compact discs; they have replaced the scalpel in many operating theaters; and they play major roles on the battlefield with laser-guided munitions. However, one of the first practical uses of the laser was in precisely determining the orbits of satellites.
Initial experiments in ranging to satellites carrying corner-cube retroreflectors began in 1964 just a few years after the laser was invented in 1960. Satellite laser ranging (SLR) stations were built in several countries, and a number of multi-instrument satellites with retroreflectors were launched by the U.S. and other nations along with dedicated spherical satellites with no electronic instrumentation — just the retroreflectors covering the satellite’s surface. The first of these was the Laser Geodynamics Satellite, or LAGEOS. It was designed by NASA and launched in 1976. LAGEOS and the other satellites carrying retroreflectors played a significant part in NASA’s Crustal Dynamics Project (CDP). Initiated in 1979, the CDP promoted the use of SLR and very long baseline interferometry to improve our understanding of plate tectonics, the rotational dynamics of the Earth, and the structure of the Earth’s gravity field.
As a post-doctoral fellow at the Massachusetts Institute of Technology and later at the University of New Brunswick, I participated in the CDP with analyses of lunar laser ranging (LLR) data. Ranging to reflectors placed on the moon’s surface by Apollo astronauts as well as those on the Russian Lunokhod rovers was a bit more difficult than ranging to satellites given the larger distances to the reflectors and the much weaker return pulses. Among other advances, LLR was the first technique to confirm the existence of variations in the spin of the Earth with a periodicity of around 50 days.
But let’s get back to SLR. Today, thanks in large measure to the International Laser Ranging Service, ranging data is routinely collected on more than 70 satellites and lunar reflectors. Included is a growing list of GNSS satellites equipped with corner-cube retroreflectors. Laser ranging to GNSS satellites is instrumental is better modeling the orbits of these satellites. Among other benefits, better GNSS satellite orbits result in better receiver position accuracies — accuracies needed to improve monitoring of crustal strain, for example, including that associated with earthquakes.
In this month’s column, we take a look at the past, present and future of laser ranging to GNSS satellites and how laser ranging and microwave ranging are mutually beneficial. They are truly kindred spirits.
Nighttime ranging at NASA’s Next Generation SLR system at Goddard Space Flight Center, Maryland. (Credit: Felipe Hall/HTSI)
Satellite laser ranging or SLR has been an indispensable independent tool for validating the precise orbits determined for GNSS satellites using microwave pseudorange and carrier-phase observations for several decades. SLR has allowed researchers to identify several orbit-modeling issues. Adding albedo radiation pressure and antenna thrust, among other effects, into the GPS orbit model allowed them to eliminate the observed bias between microwave- and SLR-derived orbits. For the first Galileo satellites launched, SLR residuals indicated severe orbit modeling issues caused by the different shape of Galileo satellite bodies compared to those of GPS. In the future, all GNSS satellites will be equipped with laser retroreflectors, a big challenge for researchers concerning tracking scenarios and observation planning to make economic use of the ground equipment.
In this article, we will take a brief look at the history of laser ranging to navigation satellites, how that ranging has improved the accuracy of the orbits of those satellites, and what the future portends for this important contribution to space geodesy.
VALIDATION OF GNSS ORBITS
FIGURE 1. Operating principle of satellite laser ranging.
In 1964, only four years after Theodore Maiman built the first laser, the first laser echoes were obtained from NASA’s Explorer 22 satellite. SLR rapidly developed into an indispensable tool for precise orbit determination, gravity field determination, and Earth system research.
FIGURE 1 shows the principles of SLR operation. Essentially, an SLR station fires a series of laser pulses at passing satellites equipped with corner-cube retroreflectors, and the relatively few photons returned are collected by a telescope. The station electronics measures the round-trip travel times of the laser pulses. From these measurements, the coordinates of the SLR station or the satellite’s orbit can be determined.
Observations by a global network of SLR stations are coordinated by the International Laser Ranging Service (ILRS), which, like the International GNSS Service, is one of the space geodetic services of the International Association of Geodesy (IAG).
FIGURE 2. Retroreflector array on GPS Block IIA satellites SVNs 35 and 36.
Since the early 1990s, the ILRS has tracked GNSS satellites supporting the independent validation of the microwave-derived precise orbits. Two Block IIA GPS satellites, SVN35 and SVN36, were equipped with retroreflectors (see FIGURE 2) and they were routinely tracked from their launches in 1993 and 1994, respectively, until their decommissioning in 2013 and 2014 (actually, SVN36 was subsequently briefly reactivated in 2015 so data is available for that satellite until that year). Also in the 1990s, the ILRS started to track GLONASS satellites in support of the International GLONASS Experiment (IGEX-98). There is a retroreflector array on all GLONASS satellites (see FIGURE 3).
Range residuals of GPS and GLONASS satellites were studied in the early years by a number of different research groups. Most of their analyses showed a bias of about –5.5 centimeters for GPS satellite orbits derived from microwave tracking data by the IGS while the accuracy of the latter was estimated to about 5 centimeters. For GLONASS orbits, a negative bias of about –4 centimeters was identified, too. The accuracy of the orbits was, however, at the 10–15 centimeter level. These validation results supported several model improvements for GPS satellite orbits including, in particular, the handling of solar and Earth albedo radiation pressure and antenna thrust, reducing the observed SLR bias with respect to the IGS orbits to 1.3 centimeters with a standard deviation of about 2 centimeters.
“What are radiation pressure and antenna thrust?” you might ask. The photons making up the light coming directly from the sun or reflected from the Earth’s surface (albedo) impinge on a satellite and transfer some of their energy to it. Solar radiation pressure – the force due to the impact of the photons – is tiny, but its continuing presence has a strong perturbing effect on satellite orbits. Antenna thrust is also a small force. The transmission of GPS navigation signals results in a continuously acting reactive force in the radial direction acting on the satellite.
FIGURE 4. Retroreflector array on Galileo satellites (at bottom of satellite, below antenna array).
SLR also plays an essential role for calibrating improved radiation pressure models for the new satellite systems. All Galileo satellites have retroreflectors (see FIGURE 4), and the orbits of the first satellites to be launched, generated using the classical extended radiation pressure model of the Center for Orbit Determination in Europe (operating in the framework of the IGS Multi-GNSS Pilot Project or MGEX), had SLR residuals as large as 20 centimeters for passes with a small beta angle. (The beta angle is the angle between the sun and a satellite’s orbital plane.) The origin of this behavior is the elongated shape of the Galileo satellites compared to the more-or-less cubic shape of GPS satellites, causing much larger variations of the satellite cross-section exposed to the sun while orbiting the Earth. The observed SLR residuals triggered the development of improved radiation pressure models for Galileo satellites.
All BeiDou satellites are also believed to be equipped with retroreflectors (see FIGURE 5). As the estimated longitude of geostationary GNSS satellites such as those in the BeiDou constellation is highly susceptible to biases due to the small motion of the satellites with respect to the tracking stations, SLR may play an important role for precise orbit determination of this category of satellite.
FIGURE 5. Retroreflector array on BeiDou satellites.
The satellites of the Indian Regional Navigation Satellite System (IRNSS), also known as the Navigation with Indian Constellation system or NavIC, also carry retroreflectors (see FIGURE 6) and have been tracked by SLR stations. However, little publicly available microwave tracking data yet exists. Therefore, up to now, precise orbit determination heavily relies on SLR observations.
FIGURE 6. Retroreflector array on NavIC satellites.
MORE APPLICATIONS OF SLR FOR GNSS
Because GNSS is a one-way measurement technique, only pseudoranges and carrier phases can be measured, and clock synchronization is indispensable for positioning and orbit determination. Radial orbit errors can therefore be absorbed to a large degree by satellite clock corrections. For the very stable clocks on board Galileo satellites, the SLR residuals show the same behavior as the microwave-derived clock corrections indicating that the clock corrections are, in fact, caused by radial orbit errors. SLR therefore provides a way to break this correlation and to separate radial orbit errors and satellite clock corrections. This makes it possible to study and to characterize the physical behavior of onboard clocks including temperature-induced clock variations.
Separation of orbit errors and satellite clock variations is crucial when using the first two Full Operational Capability Galileo satellites, which were released into wrong orbits, for relativistic experiments. In a dual launch on Aug. 22, 2014, the two satellites were put into orbits with an initial eccentricity of 0.233 and orbit height of 19,800 kilometers due to a malfunction of the launcher third stage. With a sequence of maneuvers, the satellite orbit heights could be increased to 22,600 kilometers (compared to the planned height of 23,200 kilometers) and the eccentricity was decreased to 0.156. The satellites are, nevertheless, fully functional, and the very stable hydrogen masers on board should allow scientists to improve the uncertainty of the relativistic redshift parameter α beyond the current value determined in 1976 using the Gravity Probe A satellite. Regular SLR tracking of the two satellites plays an essential role in this experiment to separate clock variations due to orbit errors from those caused by the gravitational redshift.
Eventually, SLR may also be used as a tool for high-precision time synchronization of stable GNSS clocks combining one-way laser transmissions with two-way active laser operation, similar to the concept of the European Laser Timing experiment foreseen using the Atomic Clock Ensemble in Space (ACES) on the International Space Station and already tested for BeiDou satellites.
SLR TRACKING OF THE GNSS CONSTELLATIONS
In the near future, more than 100 GNSS satellites carrying retroreflectors will be operational. This includes GPS Block III satellites, which will carry retroreflectors starting with SV-9. Tracking the full GNSS constellation will pose a big challenge for the ILRS concerning economic use of its ground equipment. Optimized tracking scenarios and session planning strategies will be indispensable.
Already today, the ILRS regularly tracks a large number of GNSS satellites. TABLE 1 shows the number of SLR normal points from ranging to the various GNSS constellations available at the ILRS data centers since 2010. Normal points are compressed full-rate data obtained by averaging individual range measurements typically over five-minute intervals. As part of the Laser Ranging to GNSS Spacecraft Experiment or LARGE project of the ILRS, the tracking of GLONASS satellites was extended to the entire satellite constellation as shown in FIGURE 7.
FIGURE 7. Number of SLR normal points per month for GLONASS satellites.
To assess the capability of SLR for GNSS precise orbit determination based on the number of tracking stations and the distribution of observations, we performed a simple simulation. The covariance analysis included observations of a single SLR station compared to networks of 6 and 17 globally distributed stations. For each station, three normal points were simulated per satellite pass for a full 24-satellite Galileo constellation: two observed at 30° rising and setting elevation angles and one at maximum elevation angle. No unfavorable weather conditions were considered and observations of different stations were assumed to be uncoordinated.
Formal errors of the determined orbits are shown in FIGURE 8 for the radial, along-track, and cross-track components. As expected, orbits determined with observations from one day’s observations by a single station reach formal errors in the few 10s of kilometers range (plot on the left in the first row). If observations from three days are used for orbit determination, the errors on the middle day reduce to about 100 meters (right, first row). The situation significantly improves if a global network of six stations is considered. Even for a single day of observations, an orbit precision of a few decimeters is reached (left, second row) while the orbit uncertainty further decreases to a few centimeters if observations from three days are used (right, second row). If, however, in an effort to reduce the number of observations per pass, only measurements at satellite culmination are acquired, the orbit precision is in the kilometer range for a six-station network and observations from one day (left, third row). If observations from three days are used, the orbit precision is at the meter level (right, third row). Using three normal points per pass for a 17-station network, the orbit precision reaches a few centimeters even within one day (left, last row) and about 1 centimeter for observations from three days (right, last row). It should be noted that the covariance analysis does not consider any systematic observation or orbit modeling error.
FIGURE 8. Formal errors of Galileo orbits in radial (red), along-track (green) and cross-track (blue) directions. First row: one SLR station, 1-day arc (left), middle of 3-day arc (right); second row: six stations, 1-day arc (left), 3-day arc (right); third row: six stations with tracking only at culmination, 1-day arc (left), 3-day arc (right); fourth row: 17 stations, 1-day arc (left), 3-day arc (right). Note the different scaling for the various plots.
This simulation is very simple and not very realistic, but nevertheless indicates the capability of precise orbit determination for GNSS satellites using a limited number of observations per station. The simulations demonstrate two facts. Firstly, even with just two or three normal points per satellite of a GNSS constellation, a significant fraction of the observation time of a station is required. Typically, a mid-latitude station can acquire about 60 normal points per day for a 24-satellite constellation, amounting to several hours of observation time per day. Secondly, the improvement in formal orbit accuracy only increases with the square root of the number of stations. More important than the number of normal points is their distribution along the orbit requiring SLR observations from several stations distributed over the globe.
These two findings make it obvious that coordination among SLR stations is indispensable for making economic use of the observing time of SLR stations while providing good coverage of normal points along all satellite orbits. To cope with weather conditions, this coordinated scheduling of GNSS SLR tracking may have to be optimized in real time.
CONCLUSIONS
SLR has played an important role in validating GNSS-derived satellite orbits for the past several decades. For new GNSS constellations and new orbit types, SLR proves to be essential for calibrating radiation pressure models and allows us to separate orbit- and temperature-induced variations of onboard clocks. Eventually, the role of SLR will become even more important by contributing to the precise orbit determination of GNSS satellites. Given the large number of GNSS satellites from several constellations equipped with retroreflectors, coordination of observation scheduling among SLR stations will be crucial for optimizing the benefit-to-cost ratio.
Concerning the distribution of SLR observations over the constellations, the following conclusions may be drawn:
For the validation and calibration of radiation pressure models, it is sufficient to acquire well-distributed observations along the orbit of one satellite for each constellation block type for a range of solar beta angles, that is, of one satellite block type per orbital plane.
For contributing to precise orbit products, optimally combined with microwave GNSS observations, the tracking of all satellites of a constellation is needed. This requires a coordinated scheduling of observations among SLR stations.
For determination of the gravitational redshift parameter using the two Galileo satellites in eccentric orbits, good coverage of the orbits of both satellites is required (as long as the satellites run on one of the onboard hydrogen maser clocks).
For BeiDou and NavIC geostationary satellites, SLR coverage is needed for all satellites to resolve biases in the microwave tracking technique.
In the long term, SLR observations could contribute, together with microwave observations, in providing operational high-precision orbit products for all GNSS constellations jointly by the ILRS and the IGS in the framework of the IAG’s Global Geodetic Observing System.
ACKNOWLEDGMENTS
This article is based on the invited paper “Ranging the GNSS Constellation” presented at the 20th International Workshop on Laser Ranging held in Potsdam, Germany, Oct. 10–14, 2016. Figure 1 was adapted from an image in “Expert Advice: Laser Reflectors to Ride on Board GPS III” published by GPS World. GPS, Galileo, BeiDou and NavIC retroreflector images obtained from the ILRS. The GLONASS retroreflector image was obtained from ISS Reshetnev. Opening photo: Nighttime ranging at NASA’s Next Generation SLR system at Goddard Space Flight Center, Maryland (Credit: Felipe Hall/HTSI).
URS HUGENTOBLER is a professor of satellite geodesy at the Technische Universität München, Germany, and head of the Satellite Geodesy Research Facility in the Institute for Astronomical and Physical Geodesy. He is also a former chair of the IGS Governing Body. His research activities include precise positioning using GNSS, precise orbit determination and modeling, reference-frame realization, clock modeling and time transfer, using both the legacy and new satellite systems. Hugentobler obtained his Ph.D. from the University of Bern, Switzerland, in 1997.
FURTHER READING
Author’s Conference Paper
“Ranging the GNSS Constellation” by U. Hugentobler, presented at the 20th International Workshop on Laser Ranging held in Potsdam, Germany, Oct. 10–14, 2016.
Early Work on Satellite Laser Ranging
“Satellite Laser Ranging: Current Status and Future Prospects” by J.J. Degnan in IEEE Transactions on Geoscience and Remote Sensing, Vol. GE-23, No. 4, July 1985, pp. 398–413, doi: 10.1109/TGRS.1985.289430.
“Reflection of Ruby Laser Radiation from Explorer XXII” by H.H. Plotkin, T.S. Johnson, P. Spandin and J. Moye in Proceedings of the IEEE, Vol. 53, No. 3, March 1965, pp. 301–302, doi: 10.1109/PROC.1965.3694.
Early Work on GPS Orbit Modeling
“Extended Orbit Modeling Techniques at the CODE Processing Center of the International GPS Service for Geodynamics (IGS): Theory and Initial Results” by G. Beutler, E. Brockmann, W. Gurtner, U. Hugentobler, L. Mervart, M. Rothacher and A. Verdun in Manuscripta Geodaetica, Vol. 19, 1994, pp. 367–386.
The International Laser Ranging Service
“The International Laser Ranging Service” by M.R. Pearlman, J.J. Degnan and J.M. Bosworth in Advances in Space Research, Vol. 30, No. 2, July 2002, pp. 135–143, doi: 10.1016/S0273-1177(02)00277-6.
SLR Tracking of GNSS Constellations
“Satellite Laser Ranging to GPS and GLONASS” by K. Sósnica, D. Thaller, R. Dach, P. Steigenberger, G. Beutler and D. Arnold in Journal of Geodesy, Vol. 89, No. 7, July 2015, pp. 725–743, doi: 10.1007/s00190-015-0810-8.
“IRNSS Orbit Determination and Broadcast Ephemeris Assessment” by O. Montenbruck, P. Steigenberger and S. Riley in Proceedings of ION ITM 2015, the 2015 International Technical Meeting of The Institute of Navigation, Dana Point, California, Jan. 26–28, 2015, pp. 185–193.
“Initial Results of Precise Orbit and Clock Determination for COMPASS Navigation Satellite System” by Q. Zhao, J. Guo, M. Li, L. Qu, Z. Hu, C. Shi and J. Liu in Journal of Geodesy, Vol. 87, No. 5. May 2013, pp. 475–486, doi: 10.1007/s00190-013-0622-7.
“Contribution of SLR Tracking Data to GNSS Orbit Determination” by C. Urschl, G. Beutler, W. Gurtner, U. Hugentobler and S. Schaer in Advances in Space Research, Vol. 39, No. 10, 2007, pp. 1515–1523, doi: 10.1016/j.asr.2007.01.038.
Effect of Radiation Pressure on GNSS Satellite Orbits
“CODE’s New Solar Radiation Pressure Model for GNSS Orbit Determination” by D. Arnold, M. Meindl, G. Beutler, R. Dach, S. Schaer, S. Lutz, L. Prange, K. Sósnica, L. Mervart and A. Jäggi in Journal of Geodesy, Vol. 89, No. 8, Aug. 2015, pp. 775–791, doi: 10.1007/s00190-015-0814-4.
“Enhanced Solar Radiation Pressure Modeling for Galileo Satellites” by O. Montenbruck, P. Steigenberger and U. Hugentobler in Journal of Geodesy, Vol. 89, No. 3, March 2015, pp. 283–297, doi: 10.1007/s00190-014-0774-0.
“Impact of Earth Radiation Pressure on GPS Position Estimates” by C.J. Rodriguez-Solano, U. Hugentobler, P. Steigenberger and S. Lutz in Journal of Geodesy, Vol. 86, No. 5, May 2012, pp. 309–317, doi: 10.1007/s00190-011-0517-4.
“Test of the Gravitational Redshift with Stable Clocks in Eccentric Orbits: Application to Galileo Satellites 5 and 6” by P. Delva, A. Hees, S. Bertone, E. Richard and P. Wolf in Classical and Quantum Gravity, Vol. 32, No. 23, 2015, doi: 10.1088/0264-9381/32/23/232003.
On April 6, Russia commissioned a new GLONASS ground station in Managua, Nicaragua — its first in Central America.
The station was created under a Jan. 26, 2012, agreement between the two countries to cooperate in the exploration and use of outer space for peaceful purposes.
GLONASS consists of 24 satellites orbiting at a medium height of 19140 km in three planes with inclination of 64.8 degrees. Access to navigation signals of GLONASS is provided to consumers across the globe free of charge and without restriction.
The station in Nicaragua is part of a global network that will monitor performance of all GNSS — GLONASS, GPS, Galileo and BeiDou.
Monitoring stations help ensure interoperability of the GLONASS system with other navigation systems and improves accuracy and reliability. It will also contribute to a precise global coordinate system, which is a key element of many international scientific programs, such as a global Earth monitoring program implemented by the International Association of Geodesy.
The information received by the station will be used for the Nicaraguan side of the self-control characteristics of navigation systems, refinement of its national system of coordinates, as well as for high-precision navigation services in order to improve the efficiency of agriculture, used in construction, aviation, navigation and scientific research.
The opening ceremony of the station was attended by a delegation of the government space state corporation Roscosmos, headed by the general director Igor Komarov, Russian ambassador to Nicaragua Andrew Buda, representative of the president of Nicaragua Laureano Ortega, and other officials.
“Our cooperation with Russia shows the kind of results we can achieve together in a short time, when there is the political will and technical capacity,” Ortega said. “This station is very important for Nicaragua and Central America as a whole. The use of the data will enrich all areas of industry and science of Nicaragua. ”
“The history of our cooperation in space activities has wonderful traditions, but today what we are doing together is a serious new step, important for modern society,” Komarov said. “In Nicaragua, the ground station will start operating GLONASS global navigation system, which embodies the most advanced technologies in the creation of navigation systems that will significantly improve the navigation performance of work on a global scale. This is the first GLONASS station in Central America, and it starts to work in Managua, which indicates the level of understanding of Russia and Nicaragua. “
